Electrical power converters, including DC-DC, DC-AC, and AC-DC are critical components in almost all forms of modern electrical systems. The vast majority of modern isolated power converters are based around traditional magnetic transformers that utilize magnetic fields to provide transformation of electrical voltage and current. Recent advances in piezoelectric transformer technology have led to alternate methods for provision of electrical power transformation through mechanical vibration. Piezoelectric transformers provide some significant benefits over their magnetic counterparts, such as increased power density, low-profile form factors, and reduced electromagnetic emission and susceptibility, while their benefits do not come without cost.
Piezoelectric transformers are inherently resonant devices, or more generally, they must be operated near their resonant frequency in order to properly function. This necessity, when combined with their highly reactive nature, creates a unique set of challenges when utilizing them within electrical power conversion circuits. First, their resonant frequency is not a static value; the resonant frequency of the device will shift as a function of numerous electrical and environmental parameters, such as device temperature, electrical load, and mounting conditions. Their power transformation efficiency is also based on a complex relationship between electrical load and operation frequency, so in order to maximize electrical efficiency some form of frequency tracking or compensation is typically implemented within converter circuitry. Current trends in the art have demonstrated numerous methods to provide such frequency compensation, typically through electrical parameter measurements of the piezoelectric transformer input/output signals in order to derive an awareness of the operational state of the device with respect to its current resonant frequency. This has previously been accomplished by utilizing self-excitation or self-oscillating circuits where the operation frequency of the device is derived from its own output, while other methods utilized phase-locked loops to continually adjust the operation frequency to maintain an operating condition directly at or near the resonant frequency of the device. Predetermined frequency sweeps to actively search or scan for the desired frequency have also been previously demonstrated. Each of these methods effectively result in a frequency controlled or compensated system.
It is important to acknowledge at this point that the electrical voltage gain and impedance of piezoelectric transformers are significantly impacted by their respective operation frequency. Typically, the maximum gain and minimum electrical impedance occurs at the resonant frequency of the device, and the maximum efficiency point occurs slightly after the devices' resonant frequency. Generally, the gain of a piezoelectric transformer reduces as the operation frequency deviates from the devices current resonant frequency, thus creating an inherent relationship between the operation frequency and gain of the device.
The majority of electrical power converters contain some level of voltage or current regulation in order to be effective in application. Current trends in both magnetic and piezoelectric transformer applications are to provide this regulation by some type of electrical modulation, effectively controlling, or throttling, the power in the transformer circuit. This power throttle is then typically responsive to the error, or difference, between some variety of electrical parameter measurement and fixed or dynamic reference, resulting in a system with a stabilized or regulated output.
Recent related art has demonstrated numerous methods for provision of either frequency controlled or regulated output piezoelectric transformer based converter systems, but few have demonstrated effective methods to simultaneously provide both. In order to fully realize the benefits of piezoelectric transformer based converters, dynamic closed-loop control of the operation frequency and output regulation must be simultaneously implemented on a continuous basis. The frequency dependent gain characteristics of piezoelectric transformers expose significant challenges when attempting to simultaneously maintain an optimal operation frequency and a voltage or current regulated system. Previous trends in the art have accomplished this by providing a limited level of control over one of the two parameters, such as utilizing a discontinuous style control of the output by effectively rapidly switching the system on and off, or utilizing a self-excited frequency generation scheme. While this does enable a form of control over both frequency and regulation its overall effectiveness is typically limited to specific applications.
A piezoelectric power converter system with both high-accuracy continuous frequency control, to maximize transformer efficiency, while simultaneously enacting high-speed continuous output regulation would demonstrate an exceptionally attractive candidate for full realization of the benefits of piezoelectric transformer based power conversion. Piezoelectric based converters have been previously targeted to niche applications, such as power supplies for cold-cathode fluorescent lamps and other high-voltage applications where limited regulation and frequency control are needed. A piezoelectric transformer based converter with high-accuracy simultaneous two-parameter control of both operation frequency and output voltage or current regulation has the potential to provide a direct replacement for a wide variety of magnetic based power converters. Such a system would demonstrate a viable alternate to traditional power converter approaches, while providing significant advancements in power conversion efficiency, electrical performance, size, and power density.